1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor and a method and an apparatus of forming an electrophotographic image. In particular, the present invention relates to an electrophotographic photoreceptor that contains a p-type semiconductor fine particulate material in an amount smaller than that of a commonly used CuAlO2 fine particulate material, that maintains a low residual potential over a long period of time, that causes no residual image on the photoreceptor surface even after repeated use, that causes no blurred image even under high-temperature and high-humidity conditions, and that exhibits excellent durability, and a method and an apparatus of forming an electrophotographic image.
2. Description of Related Art
A typical electrophotographic image-forming apparatus includes an electrophotographic photoreceptor (hereinafter also referred to simply as “photoreceptor”), such as an inorganic photoreceptor or an organic photoreceptor.
In an “electrophotographic” image-forming process, the photoconductive photoreceptor is charged in a dark place by, for example, corona discharge and is then exposed to selectively dissipate only the charges on the exposed portions and to produce an electrostatic latent image, and the latent image is developed into a visualized image with a toner containing a resin material and a colorant, such as a dye or pigment.
The organic photoreceptor is more advantageous than the inorganic photoreceptor in terms of high selectivity on a photosensitive wavelength range, high film formability, high flexibility, high transparency of the resultant film, high mass productivity, low toxicity, and low production cost. Thus, most current photoreceptors are organic photoreceptors.
Such a photoreceptor is required to have high durability in view of, for example, a reduction in environmental load, an improvement in productivity, and a reduction in production cost.
A recent technique for enhancing the durability of the photoreceptor involves addition of a filler (inorganic fine particles) to a surface protective layer, to strengthen the layer for a prolonged service life.
Unfortunately, a common filler used in this technique is a fine particulate electron transporting n-type semiconductor, such as aluminum oxide, titanium dioxide, or tin oxide, and the filler leads to an increase in residual potential after repeated exposure, resulting in failure to achieve stable image formation. This is probably attributed to the fact that the fine particulate n-type semiconductor in the surface protective layer has no hole transporting ability, and thus holes are trapped at the interface between the charge transporting layer and the surface protective layer and at grain boundaries in the surface protective layer, resulting in ineffective cancellation of negative charges on the surface of the photoreceptor.
Although addition of a charge transporting material (CTM) to the surface protective layer in combination with the fine particulate n-type semiconductor can solve the problems involved in image formation, the charge transporting material serves as a plasticizer, leading to a reduction in surface hardness. Thus, much difficulty is encountered in achieving both the hole transporting ability and high strength of the surface protective layer.
A known technique for solving such a problem involves addition of a fine particulate p-type semiconductor as a filer having hole transporting ability. The fine particulate p-type semiconductor is, for example, CuAlO2 fine particles (see, for example, Japanese Unexamined Patent Application Publication Nos. 2013-130603 and 2014-021133).
Addition of CuAlO2 fine particles can achieve both high strength and necessary hole transportation. For higher-speed printing, the photoreceptor is required to have improved hole transporting ability. If the amount of CuAlO2 fine particles added to the surface protective layer is increased for further improvement of hole transporting ability, the number of hydroxy groups derived from the fine particles is also increased, and thus moisture in air is adsorbed onto the hydroxy groups, leading to a reduction in resistance, resulting in blurred images under high temperature and high humidity conditions. Hydrophobization of the entire hydroxy groups with a silane coupling agent, which may solve the aforementioned problems, is very difficult to perform.